The present invention relates to a composition for a tread rubber of tires having a low rolling resistance and an improved wet grip characteristic, and more particularly to a rubber composition containing a synthetic isoprene rubber having specified 3,4-bonding content and intrinsic viscosity.
In recent years, reduction of running fuel cost of automobiles such as passenger cars is strongly demanded in the automobile industry from the viewpoint of the economical efficiency and energy saving. There is mentioned the rolling resistance of tires as one factor participating in such a running fuel cost. It is known that the rolling resistance of tires is caused by energy losses in rubbers and codes of respective portions of the tires due to repeated deformation of the tires, and in particular the energy losses in a tread rubber largely participate in the rolling resistance. Therefore, in order to reduce the rolling resistance caused by the tread rubber, it is necessary to reduce the energy losses due to various repeated deformations, i.e. compressive deformation, bending deformation and shearing deformation, that the tread rubber incurs in running. Seeing it from the dynamic viscoelasticity of a rubber, this means that it is necessary to reduce the loss compliance [E"/(E*).sup.2 in which E" is loss modulus and E* is complex modulus] with respect to the energy loss due to compressive deformation and to reduce the loss modulus (E") with respect to the energy losses due to bending and shearing deformations. Therefore, the use of natural rubber and synthetic cis-isoprene rubber which have generally a low loss compliance and a low loss modulus is advantageous in rolling resistance. However, these rubbers have the defect of being poor in the frictional characteristic on a wet road (hereinafter referred to as "wet grip" characteristic) which is the most important characteristic for tires of automobiles.